Yumeko Jabami is the main protagonist of Kakegurui. She is a transfer student at Hyakkaou Private Academy and the classmate of Ryota Suzui and Mary Saotome. Yumeko's family is aligned with the family of Kirari Momobami and challenges her for Hyakkaou Private Academy.
Yumeko is a very attractive pale-skinned girl with long black hair that is styled in a hime-cut and brown eyes. In the anime and manga, her eyes are sometimes seen red whenever she becomes excited. She wears the standard female Hyakkaou Private Academy issued uniform; a red blazer with black trim lining the cuffs and collar, a white button-up dress shirt, a dark pleated skirt, a tie and black/grey colored stockings. As well as the academy's issued footwear, brown colored loafers with black soles. She also sports red lipstick and has red painted fingernails (sometimes seen pink). She wears a ring around her left thumb.
It is hinted that she has low stamina as she was having difficulty in climbing five floors worth of staircases.
Yumeko is shown to be a very upbeat and charismatic character, consistently attracting the affection or interest of those around her. Not only is she able to befriend those who previously disliked her, she is able to catch the attention of the entire student body. She does just that on her very first day of school and manages to continue doing so on nearly a daily basis. Every time she notices an opponent cheating in games, her button gets switched to a bloodthirsty side of her, usually seen when her eyes turn red and she frightens her opponents.
Though she is typically very friendly, she abstains from acting this way when she uses her perceptiveness to expose her classmates' cheating. She is capable of doing this even after only a couple of rounds of gambling, as she is easily able to decipher what strategies and tricks they are using to skew the game in their favor. While doing this, she may unabashedly condemn the other player and their ways, preferring honesty over politeness in such cases.
Despite the disadvantage, she faces when others cheat, she does not concede. This is largely due to the fact that she does not seem to experience fear or anxiety from taking risks. In fact, it gives her a thrill. Because of this, she tends to take high-risk bets in gambles, even if she has nothing to rely on but luck. Though this can sometimes end in her loss, she is emotionally unaffected by the results; she simply uses her loss or gains a means to further indulge her gambling habit.
She seems to be fairly aware of her eccentricities and that her way of gambling doesn't follow normal conduct. As such she rejoices when people support her or encourage her, especially those close to her, greatly valuing their friendship, albeit in her own ways. She rationally understands that her behaviour is immoral as she confesses to feel guilt for not feeling guilt over the people she has ruined through gambling.
Kakegurui - Compulsive Gambler
Yumeko transfers to Hyakkaou Private Academy and is shown around the school by her classmate Ryota. Ryota tells her about the school which she seems to be interested in. She became popular in her class as she is surrounded by her fellow male classmates. She is then challenged by Mary to a gambling match which she accepts. She is shown to be losing after betting the match after betting one million yen during the first two rounds. Yumeko then asks Mary for another round to which Mary agrees, when asked how much she's willing to bet, Yumeko reaches into her school bag and produces stacks of yen that amount to 10 million, especially when Yumeko puts 10 million yen on one round. Yumeko then points out that Mary rigged the game in her favor by having 21 out of 30 students place rigged cards that would tip the game in her favor. Yumeko reveals that she noticed this patterns from the beginning and reveals Mary's strategy to her. Yumeko wins the gambling match leaving Mary with a debt of 8.8 million yen which she tells Mary that her debt is already paid as their gambling match proved to be a fun ordeal. After that, Yumeko leaves her classroom were Ryota calls out to her. He asks her how she knew about Mary's rigging of the match to which she reveals to him how she deduced everything. She later gives him money to pay his debt that he has to Mary as thanks.
The next day it is seen that Yumeko and Ryota are now friends. They enter their classroom to find Mary's desk vandalized with the entire class smiling menacingly. Mary blames it on Yumeko because of her loss yesterday before lurking away to earn the money back. Ryota takes Yumeko to the cafeteria where he explains the oppressive structure of the school, the student council, and their gambling prowess, increasing her already growing love and excitement towards the school. Yumeko is then invited to play Concentration against Itsuki Sumeragi. Yumeko plays a close game but loses the 20 million yen given as a trial. Yumeko demands a rematch and Itsuki accepts under the condition that should Yumeko lose, Itsuki can remove her fingernails and toenails to add to her nail collection. Yumeko realizes her opponent was cheating and wins on the very first turn by remembering every card's patterns on the back. While Itsuki is in shock, Yumeko confronts her on the cheating and starts spewing out her excitement mixed with anger asking for another game where they both bet their nails. The confrontation caused Itsuki to cry and decline Yumeko's desire making the latter disappointed.
Yumeko's surname: Jabami（蛇喰）can be translated to have any number of meanings; as the first kanji （蛇）can mean "snake/serpent" or "hard drinker" and the second kanji（喰）translates to "eat," "drink," or "to receive a blow." The likeliest translation of her surname is snake eater. Her first name: Yumeko（夢子）translates to "dream child."
- In the first volume, it was revealed that she lives alone and her parents have passed away. She also has an older sister that is currently being hospitalized.